In recent years, small mobile terminals, such as smartphones, are in widespread use, and in accordance therewith dialogue systems allowing natural speech inputs have received attention. The dialogue systems allowing natural speech inputs interpret a user's intention without the need for users to adapt their speech to the systems; thus, users do not have to use predefined phrases, and they can give instructions to the system with natural expressions. In such dialogue systems, if conditions for a search performed during a past dialogue are changed to perform a new search, it is necessary to undo a dialogue.
To undo a dialogue, there is a method of selecting a dialogue state to be applied depending on whether or not the search conditions are exclusive, without a need for a user to say “undo”.